Electric power steering provides a steering assist to a motor vehicle driver as the driver turns the steering wheel in either direction of rotation. The electric motor of the electric power steering (EPS) system which serves to assist the steering by the driver can be connected to the rack of the steering system (an REPS system) or be connected to the steering column (a CEPS system), which are exemplified at FIGS. 1A and 1B.
FIG. 1A depicts an example of a CEPS system. A motor vehicle 40 is provided with a steering column electric power steering system 24 which may comprise a conventional rack and pinion steering mechanism 36, which includes a toothed rack (not shown) and a column pinion gear (not shown) of a gear box 52. As the steering wheel 26 is turned, an upper steering shaft 29 turns a lower steering shaft 51 through a universal joint 34; and the lower steering shaft 51 turns the column pinion gear. Rotation of the column pinion gear moves the rack, which moves tie rods 38 (only one shown), which move steering knuckles 39 (only one shown) to turn tires 42 (only one shown).
The electric power assist is provided through a controller 16 and a power assist actuator comprising an electric motor 46. The controller 16 receives electric power from a vehicle electric power source 10 through a line 12, a signal representative of the vehicle velocity on line 14 and column pinion gear angle from a column rotational position sensor 32 on line 20. As the steering wheel 26 is turned, a torque sensor 28 senses the torque applied to steering wheel 26 by the vehicle operator and provides an operator torque signal to controller 16 on line 18. In addition, as the rotor of the electric motor 46 turns, rotor position signals for each phase are generated within the electric motor and provided over bus 30 to the controller 16. In response to the vehicle velocity, operator torque, column pinion gear angle and rotor position signals received, the controller 16 derives desired electric motor phase currents and provides such currents through a bus 22 to the electric motor 46, which supplies torque assist to steering shaft 29 through worm gear 47 and motor pinion gear 48. Details hereof are described in U.S. Pat. No. 5,982,067. An example of an embodiment of the controller 16 is described in U.S. Pat. No. 5,668,722.
FIG. 1B depicts an example of an REPS system. The rack electric power steering system 60 comprises a conventional rack and pinion steering mechanism 62, which includes a toothed rack 64 which is connected to the tie rods (not shown) for directing the turning of the tires (not shown). The steering column has a lower assembly 66 having a column pinion gear 68 which is meshed with the teeth 70 of the toothed rack 64 so that turning of the steering column applies a torque at the toothed rack that results in the toothed rack translating left or right, depending on the direction of the turning of the steering column. The electric motor 72 of the electric power steering system is connected (by gearing, belt, etc.) to a ballscrew gear box 76. The electrical operation is as generally described with respect to FIG. 1A, as it is adapted to the configuration of FIG. 1B.
Under normal operating conditions, the electric power steering motor responsively assists the effort of the driver at the steering wheel to effect turning of the tires. However, an electric power steering system over-speed condition may arise, for example, if the motor vehicle is moving relative to an object and a tire is struck by the object, wherein the over-speed is the result of the motor vehicle speed relative to the struck object causing a rapid turning of the tire, and through the tie rods, back-driving the rotating components of the electric power steering system. In such a situation, the rotational speed of the electric power steering rotating components may become sufficiently excessive (an over-speed condition) that, at an abrupt end of travel event, possible damage to the components of the power steering system, such as for example the tie rod connections, the steering column, the I-shaft, the rack and pinion, etc., could occur, due to the large rotational inertia of the power steering rotating components that is present at the abrupt end of travel.
Therefore, what remains needed in the art is a mechanism that can prevent power steering system damage in the event of an over-speed condition of an electric power steering system.
With regard to braking devices, all drum/shoe brakes systems used on motor vehicles for most of the past 70/80 years use “energizing” principles. Also, energizing brakes of a disc nature are found on agricultural tractors from the 1940's through the 1980's which employ a ball ramp energizing mechanism.
An energizing brake uses the friction developed between the rotating surface and the braking surface to actually generate more force between the rotating surface and the braking surface. The energizing action continues in an upward amount until the friction coefficient begins to decrease (brake fade) or the rotating surface stops. Either way, the energy input from the rotating surface is gone, so the brake disengages. The actuation mechanism is one controlled by humans for the purpose of controlling a vehicle or machine, wherein at the onset of motor vehicles, there were no power boosted brakes from hydraulics or vacuum, as we have today. So, it was desired to acquire “brake boost” from the mass of the moving object by using energizing brakes, which delivered a much higher amount of brake torque than what was possible with human applied force only. For the energizing brake, the human applied enough force to engage the brake, but the energizing function is what did the majority of the work to stop the vehicle.
Accordingly, what remains needed in the art is a mechanism to use the energizing principle to keep the brake very small and light while achieving a high amount of torque to prevent the over speed situation and using centrifugal force to actuate the brake in the first place.